Even if his soul had ascended to a higher level by sacrificing countless lives, he still wouldn't abandon his body or forge a new one, which in his prior state he was capable of. A body born from the purest of his blood, capable of greater feats than his old body could ever do. But if he does that, the curse energy that flows in his body would cease to exist. So going back to the old body was important.
And even if it can't compare to the peak of some Knight bloodlines, it was still a genuine Grand Master capable of creating a greater number of blood summons and handling a stronger level of Curse.
'Ah, this new information, this is what I lacked.' Once he returned to his old body, Rosso experienced a new flood of changes that started with knowledge that made it easier for him to control the curses as well as a new mutation in his eyes that wasn't even noted in the notes.
